Load lifting devices having load carrying members suitable for engaging and carrying a load have been in use for many decades. Typically, the load lifting device (masts, booms, and lifting linkages) is mounted on a vehicle and supports the load carrying members (forks and the like) for elevational movement relative to the vehicle. Some examples of vehicles of this type include lift trucks, telescopic material handlers, and track and wheel loaders. The load carrying members, which includes material handling forks, are mounted on the load lifting device and elevationally movable for retrieving and depositing loads at various elevational locations.
In order to lift a load, the load lifting device must be positioned so that the load carrying members are disposed beneath the load. The load carrying members are then elevated until the load is engaged by and fully supported thereon. During this loading process, there is a strong potential for the load to become skewed, pitched, and tipped relative to the load carrying members resulting in the load not being squarely carried on the load carrying member. The load not being accurately positioned on the load carrying member would adversely effect material handling and load transferring operations. This is particularly important in automated manufacturing and storage systems where driverless automatic guided vehicles are used to transport loads. Therefore, there is a need to sense when a load is squarely and properly positioned on the load carrying members so that the material handling function may be carried out with a maximum amount of efficiency and accuracy.
One attempt to solve a portion of this problem is taught in Japanese Patent Publication No. 61-15040, 1986 to Kabushiki Kaisha Komatsu Seisakusho, dated April 22. In this publication, a mechanical touch switch is provided on a load carrying member of a lift truck lift mast assembly adjacent a juncture of connection between the vertically and horizontally extending portions of the load carrying member. This, however, is an unsatisfactory solution for several reasons. Since the switch is mechanical and requires physical movement of a mechanical actuator to actuate the switch, there is the potential for switch failure due to excessive forces being applied to the switch and the mechanical switch actuator by the load.
Also, the mechanical switch actuator is frequently subjected to cyclical forces caused by loading and unloading of the load. Thus, improper switch adjustment due to wear, bending, movement and the like occurs. This causes improper switch operation and results in inaccurate, incorrect, and erroneous sensing of the actual position of the load on the load carrying members. As a result, placement of the load during load transfer operations is inaccurate and requires frequent adjustment of the switch and switch actuator.
The switch of the above-noted Japanese Patent is connected to the load carrying member adjacent the juncture of connection of the first and second end portions of the load carrying member so that the switch will be closed whenever an end portion of the load nearest the juncture of connection of the load carrying members horizontally and vertically oriented end portions (first and second end portions) contacts the mechanical switch actuator. This assumes that when the end portion of the load adjacent the switch is properly positioned that the entire load is properly positioned and squarely at rest on the second end portion of each of the load carrying members. This is, of course, a false assumption since the load may be skewed and/or tipped and/or tilted relative to the second end portions of the load carrying member while making contact with the mechanical switch actuator. Therefore, no positive and accurate sensing arrangement has been provided to sense when a load is squarely at rest on the load carrying members second end portion and closely adjacent the first end portion of the load carrying member.